Investigators are trying to determine whether Andrew Joseph Stack, the man identified by authorities as the pilot of the plane, is the same person as "Joe Stack," the name signed to the online rant that warns, "Desperate times call for desperate measures."

The wife of the suicidal Texas pilot who slammed his plane into an Austin office building called the attack today an "unimaginable tragedy."

Sheryl Stack, who was mentioned in the hate-filled suicide note thought to have been left by her husband, issued a statement extending "my sincerest sympathy to the victims and their families."

"Words cannot adequately express my sorrow or the sympathy I feel for everyone affected by this unimaginable tragedy," she said.

Sheryl Stack said she would not answer any questions because of the ongoing investigation.

The sheer volume of flames and smoke pouring from the Austin office building after A. Joseph Stack slammed into with his plane has prompted authorities to investigate whether he had some kind of explosive on board, sources told ABC News.

Play

null

Stack, 53, topped off his single engine Piper Cherokee with fuel before crashing into the IRS offices in a kamikaze mission designed to punish the government he believed wronged him.

The full tank of fuel is believed to have contributed to the force of the explosion and subsequent fire, which investigators believe was probably a deliberate tactic by Stack.

Despite the spectacular crash and fire that left the seven-story building a blackened hulk, only Stack and one other person are believed to have died. Officials have not yet identified the second victim, a federal employee -- but a man told ABC News Huntsville, Ala., affiliate WAAY-TV that the dead man was his brother, Vernon Hunter, 61, an IRS collection agent.

Play

Suicide by Plane

"He took my baby brother," said Harold L. Jackson, visibly welling up. "Oh boy, I don't want to break up. ... I can't do that. As a Christian person I have to say 'Forgive us as we forgive those,' and I have to forgive that man for doing that. But its very difficult."

Hunter, the youngest of five brothers, was adopted as a baby and kept his birth family's name, Jackson told WAAY, but that did not lessen the bond.

"We called ourselves the Jackson 5 -- the other Jackson 5," Jackson said of Hunter and his other brothers.

Stack's father-in-law, Jim Cook, said there had been worry in Stack's family very recently about Stack's state of mind regarding his tax problems.

"She had a bit of a problem with him," Cook said, referring to his daughter, Sheryl. "And evidently, in the last few days ... it boiled in his brain. She informed us that she thought he was crazy."

He said his son-in-law had been "a good guy," and seemed normal when he saw him at Christmas time.

"Everything was fine," Cook said. "We went out and ate. We spent time in their house. They have a beautiful home."

Sheryl Stack did not address the suicide note investigators believe her husband wrote -- a lengthy rage-filled diatribe against the IRS, President George W. Bush and his own accountant.

Play

null

She instead asked for "privacy and the personal space we need to get beyond the events of yesterday."

The suicide note was posted Thursday right around the same time as the 10 a.m. crash. It was signed "Joe Stack (1956-2010.)"

The note was titled "Well Mr. Big Brother IRS Man … take my pound of flesh and sleep well." It detailed years spent working and paying taxes, but not reaping the benefits of what he considered to be a functional government.

"I choose to not keep looking over my shoulder at 'big brother' while he strips my carcass, I choose not to ignore what is going on all around me, I choose not to pretend that business as usual won't continue; I have just had enough," the note read.

Play

Raw Video: Austin Texas Plane Crash, Crews Fight Building Fire

Records show two of Stack's software companies were suspended by the state tax board.

"I can only hope that the numbers quickly get too big to be white washed and ignored that the American zombies wake up and revolt; it will take nothing less," he wrote.

Forensic psychiatrist Dr. Michael Welner told "Good Morning America" that while it may appear that Stack simply snapped under the weight of tax debt, it was clear from the suicide note that this had been brewing for some time.

"This is the kind of crime that's planned for a long time," Welner said. "I wouldn't be surprised to learn that he practiced."

Suicidal Texas Plane Crash: 'Crime of Rebellion'

Calling the attack a "crime of rebellion," Welner said it was obvious that Stack had planned the crash to maximize public attention by invoking both the Oklahoma City bombing of a federal building and the 9/11 terrorism attacks.

"I think it's important to distinguish this as a spectacle murder because it's carried out in order to get on the news," he said.

The Federal Aviation Administration told ABC News that Stack did not communicate with air traffic controllers once he got in the air around 9:40 a.m. nor did he file a flight plan.

Officials told ABC News that once a man like Stack decides to plan this type of attack, there is very little they can do to stop it, especially once he's in the air and locked on a target.

Witnesses said Thursday that the plane appeared on a purposeful track, banking sharply as it flew over a major highway directly into the office building.

Beth Jones told ABCNews.com that she was headed downtown on Highway 183 when she spotted the plane overhead and immediately noticed how low it was flying.

"The plane was just coming down," she said. "He was so low you could actually see him in the plane." She said the plane flew "just straight, right in" to the complex.

Stack has been described as quiet. He had lived in his Austin neighborhood with wife Sheryl and their daughter for about three years, having moved from California.

Mechanic Dave Page, who used to work on Stack's plane in California, told ABC News that he was a hard-working guy.

"I think he pretty much, his life was work," he said. "He enjoyed doing what he did. He liked flying his airplane."